📝 Move OTEL_TRACES_SAMPLER_* feature changelog from '1.8.0-beta.1' to 'Unreleased'

The feature was added to the codebase _after_ `1.8.0-beta.1` had already been cut. It should be made available in the next release instead, which is why it should be in the "Unreleased" section of the changelog.

This was originally introduced in:
- [sdk-traces] Add support for OTEL_TRACES_SAMPLER and OTEL_TRACES_SAMPLER_ARG (open-telemetry#5448) (76cafe5)

Author: Juliano Leal Goncalves

src/OpenTelemetry/CHANGELOG.md

@@ -2,6 +2,16 @@
 
 ## Unreleased
 
+* `TracerProvider`s can now have a sampler configured via the
+  `OTEL_TRACES_SAMPLER` environment variable. The supported values are:
+  `always_off`, `always_on`, `traceidratio`, `parentbased_always_on`,
+  `parentbased_always_off`, and `parentbased_traceidratio`. The options
+  `traceidratio` and `parentbased_traceidratio` may have the sampler probability
+  configured via the `OTEL_TRACES_SAMPLER_ARG` environment variable.
+  For details see: [OpenTelemetry Environment Variable
+  Specification](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/configuration/sdk-environment-variables.md#general-sdk-configuration).
+  ([#5448](https://github.com/open-telemetry/opentelemetry-dotnet/pull/5448))
+
 ## 1.8.0-beta.1
 
 Released 2024-Mar-14
@@ -81,16 +91,6 @@ Released 2024-Mar-14
   Specification](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/configuration/sdk-environment-variables.md#exemplar).
   ([#5412](https://github.com/open-telemetry/opentelemetry-dotnet/pull/5412))
 
-* `TracerProvider`s can now have a sampler configured via the
-  `OTEL_TRACES_SAMPLER` environment variable. The supported values are:
-  `always_off`, `always_on`, `traceidratio`, `parentbased_always_on`,
-  `parentbased_always_off`, and `parentbased_traceidratio`. The options
-  `traceidratio` and `parentbased_traceidratio` may have the sampler probability
-  configured via the `OTEL_TRACES_SAMPLER_ARG` environment variable.
-  For details see: [OpenTelemetry Environment Variable
-  Specification](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/configuration/sdk-environment-variables.md#general-sdk-configuration).
-  ([#5448](https://github.com/open-telemetry/opentelemetry-dotnet/pull/5448))
-
 ## 1.7.0
 
 Released 2023-Dec-08